Injection of aqueous reagents including ammonia and urea is widely applied in the practice of Selective Non-Catalytic Reduction (SNCR) and Selective Catalytic Reduction (SCR) processes for the reduction of nitrogen oxide emissions from lean burn combustion sources. SCR has been applied to both mobile and stationary diesel engines as well as gas turbines and boilers. Products such as the XNOx™ SCR system marketed by Tenneco are now widely applied to commercial and off road vehicles. SNCR has more traditionally been applied to larger boilers for in furnace injection of reagent into a furnace cavity at an optimum temperature window of 1700-2200 F.
In certain SCR applications, the aqueous reagent is injected directly into the hot exhaust gases where the enthalpy of the exhaust gas will vaporize and decompose the reagent to ammonia gas upstream of the catalyst. U.S. Patent Application Publication Nos. 20130152470 to Lindemann, et al. and 20140099247 to Jangiti et al. describe several practical approaches to direct injection of aqueous reagents for SCR. In these applications, the duct dimensions, temperature, residence time, and quantity/type of reagent injected can impact the effectiveness of injection and distribution of reagent across the catalyst face. In larger SCR applications, an ammonia injection grid (AIG) is typically used to provide multiple points of injection of a gaseous reagent across a duct at a location upstream of an SCR catalyst. In SNCR applications, multiple levels of multiple injectors are typically used to distribute reagent across the furnace for decomposition and chemical reaction with NOx at high gas temperatures.
In some SCR applications, an aqueous based reagent is first decomposed to a gas before being injected into the exhaust duct. The reagent decomposition is accomplished by using a heated vaporizer reactor or a decomposition duct into which the reagent is injected along with a high volume of heated air and/or hot exhaust gases. In traditional urea decomposition systems, a side stream of exhaust gas or air is generally heated to a high temperature of 700-800 F and a gas flow rate of 600-1000 actual cubic feet per minute (ACFM) per gallon of reagent are used to vaporize and decompose the urea reagent.
Convention has held that wall wetting of the small exhaust duct on vehicles from urea based reagents should be avoided to prevent cool spots and the formation of urea deposits in the duct or downstream surfaces. Orientation of injectors, efforts to produce small droplet size of urea based reagents, and exhaust duct mixers have been used to minimize the wall wetting.
In SAE Paper 2006-01-0643, “Analysis of the Injection of Urea-water-solution for Automotive SCR DeNOx-Systems,” the authors, Birkhold et al., model the flow and spray/wall interaction of urea reagent injection into an exhaust duct and identify the risks of droplet impingement, localized cooling, and risk of formation of melamine complexes. Birkhold et al. recognize that wall wetting is difficult to prevent and merely develop equations to suggest the impact and design of wall interactions with reagents. They conclude that spray impingement on hot surfaces may lead to better evaporation and conversion to ammonia, but they fail to teach an improved means to avoid localized wall wetting and the formation of urea deposits.
While nitrogen oxide reduction systems are known in the art, there is no system that efficiently overcomes the urea deposit problem in urea based NOx reduction processes. Up to date, there is no effective gasification and decomposition apparatus that is suitable for use in both SNCR and SCR NOx reduction processes.
U.S. Pat. No. 7,815,881 to Lin et al. describes the use of a flue gas bypass duct for injection of urea and for conversion to ammonia for SCR. U.S. Pat. No. 7,090,810 to Sun et al. describes the reduction of NOx from large-scale combustors by injecting urea into a side stream of gases with temperature sufficient for gasification. But both of the patents are directed to large scale decomposition systems to convert urea solutions into gaseous ammonia, which require large decomposition reactors and residence times of greater than 1 second.
Commonly owned U.S. Pat. No. 8,815,197 to Broderick et al. and U.S. Pat. No. 8,591,849 to Valentine et al. describe small scale urea decomposition systems where the reagent injection rate is typically less than 10 gallons per hour (gph) with a gas flow rate in the duct at 150-3000 SCFM at a temperature greater than 700 F. The aqueous reagent is converted to ammonia gas in the decomposition duct and conveyed through the continuous duct to an ammonia injection grid placed in the primary exhaust upstream of a NOx reducing catalyst. While these patents demonstrate improvement and simplicity over the prior art, it would still be desirable to reduce the gas flow rate, to cut down on the fan/blower size and operating horsepower, as well as to minimize the supplemental heat required to raise the exhaust gas temperature to the decomposition duct.
U.S. Pat. No. 5,809,910 to Svebdssen teaches a means for NOx reduction in a large incinerator wherein rotation and turbulence in the flame of the unit are used for thermal SNCR NOx reduction at a temperature of 900-1000° C. Combustion air, recirculated flue gases, fuel, and reducing reagent are introduced into the incinerator unit with over fire air (OFA) or rotating over fire air (ROFA) through asymmetrically positioned ducts. All agents and gases in the incinerator are thereby rotated in the combustion zone.
Similarly, U.S. Pat. No. 8,449,288 to Higgins teaches the use of asymmetrical secondary air ducts to produce high velocity mass flow, turbulence and rotation with urea injection for in-furnace SNCR NOx reduction at 2000 F.
Both Svebdssesn and Higgins are not aimed at converting urea to ammonia for use in a catalytic reduction process but are directed at improving the high temperature non-catalytic SNCR NOx reduction reactions. Neither of them is preferred because of the high temperature requirement.
U.S. Pat. No. 8,501,131 to Moyeda teaches a combined SNCR and SCR NOx system that involves reagent injection in the high temperature SNCR zone and supplementary injection of reagent in the duct before the SCR catalyst. A high pressure steam is mixed with reagent to transport it through nozzles arranged on the duct wall to penetrate the gas stream and mix the reagent with the flue gases before the catalyst. There is also a reference to the use of air as a transport fluid and the suggestion that the reagent may be in a gas form if injected with air as the transport medium. However, Moyeda does not teach the vaporization and decomposition of urea to ammonia before injection of the reagent.
U.S. Pat. No. 4,246,757 to Heberling describes vaporizing fuel using a cyclonic prechamber, which is part of a combustion apparatus, for a gas turbine. The prechamber includes a plurality of highly angled vanes disposed circumferentially around the cylindrical prechamber to provide a high tangential component of velocity to combustion air in the prechamber. Liquid fuel is introduced into the prechamber and premixed with the combustion air in the cyclonic zone and continuously discharged through a throat to a combustion zone. However, Heberling neither concerns decomposition of reagent, nor addresses the issue of undesired urea deposits.
In general, the art either discloses that a high gas temperature is required for urea based NOx reduction systems or large quantities of hot gas are required to decompose urea reagent while at the same time preventing the formation of reagent deposits. The cost for heaters and fans utilizing high volumes of heated air and/or hot exhaust gases can be prohibitive for small boilers and very expensive for larger applications. It would be desirable to have a simple and cost effective method of vaporizing and/or decomposing common reagent such as aqueous ammonia or urea with a minimum of hot gas flow and without the formation of reagent deposits.